The present invention relates to a secondary power source having a low resistance, a high upper limit voltage and a large capacity. More particularly, the present invention relates to a secondary power source having a high reliability for quick charge and discharge cycles.
As electrodes for a conventional electric double layer capacitor, polarizable electrodes composed mainly of activated carbon are used for both the positive electrode and the negative electrode. The upper limit voltage of an electric double layer capacitor is 1.2 V when an aqueous electrolyte is used, or from 2.5 to 3.3 V when an organic electrolyte is used. The energy of the electric double layer capacitor is proportional to the square of the upper limit voltage. Accordingly, an organic electrolyte having a high upper limit voltage provides a high energy as compared with an aqueous electrolyte. However, even with an electric double layer capacitor employing an organic electrolyte, the energy density is as low as at most 1/10 of a secondary cell such as a lead storage battery, and further improvement of the energy density is required.
Whereas, JP-A-64-14882 proposes a secondary power source for an upper limit voltage of 3 V, which employs, as a negative electrode, an electrode having lithium ions preliminarily doped in a carbon material having a spacing of [002] face of from 0.338 to 0.356 nm as measured by X-ray diffraction. Further, JP-A-8-107048 proposes a battery which employs, for a negative electrode, a carbon material having lithium ions preliminarily doped by a chemical method or by an electrochemical method in a carbon material capable of doping and undoping lithium ions. Still further, JP-A-9-55342 proposes a secondary power source for an upper limit voltage of 4 V, which has a negative electrode having a carbon material capable of doping and undoping lithium ions supported on a porous current collector which does not form an alloy with lithium. However, these secondary power sources have had a problem from the viewpoint of the process for their production which requires preliminary doping of lithium ions.
Further, a lithium ion secondary cell is available as a power source capable of heavy current charge and discharge other than the electric double layer capacitor. The lithium ion secondary cell has characteristics such that it provides a high voltage and a high capacity as compared with the electric double layer capacitor. However, it has had problems such that the resistance is high, and the useful life due to quick charge and discharge cycles is very short as compared with the electric double layer capacitor.
Under these circumstances, it is an object of the present invention to provide a secondary power source which has quick charge and discharge capability, provides a high upper limit voltage and a high capacity and has a high energy density and which has a high charge and discharge cycle reliability.
The present invention provides a secondary power source, which comprises a positive electrode containing activated carbon and a lithium-containing transition metal oxide, a negative electrode containing a carbon material capable of doping and undoping lithium ions, and an organic electrolyte containing a lithium salt.
Now, the present invention will be described in detail with reference to the preferred embodiments.
In this specification, a negative electrode assembly is one obtained by bonding and integrating a current collector and a negative electrode composed mainly of a carbon material capable of doping and undoping lithium ions. Likewise, a positive electrode assembly is one obtained by bonding and integrating a current collector and the positive electrode. A secondary cell as well as an electric double layer capacitor is a kind of a secondary power source. However, in this specification, a secondary power source of a specific construction wherein the positive electrode contains activated carbon and the negative electrode contains a carbon material capable of doping and undoping lithium ions, will be referred to simply as a secondary power source.
With the secondary power source of the present invention, at the time of charging, at the positive electrode, in addition to the adsorption of anions of the lithium salt to the activated carbon, undoping of lithium ions from the lithium-containing transition metal oxide takes place. At the negative electrode, lithium ions will be doped in the carbon material. Here, the lithium ions to be doped in the carbon material are those from the lithium salt in the electrolyte and those undoped from the lithium-containing transition metal oxide.
Accordingly, with the secondary power source of the present invention, lithium ions can adequately be doped in the carbon material of the negative electrode, as compared with a conventional secondary power source employing a positive electrode composed mainly of activated carbon without containing a lithium-containing transition metal oxide. Therefore, an adequate amount of lithium ions can be doped in the negative electrode by charging after the positive electrode and the negative electrode facing each other with a separator interposed therebetween, are impregnated with the electrolyte, without necessity to preliminarily occlude lithium ions to the negative electrode as required in the above-mentioned conventional secondary power sources. And, the potential of the negative electrode becomes low enough, so that the voltage of the secondary power source can be made high.
With a lithium ion secondary cell wherein the positive electrode is made of an electrode composed mainly of a lithium-containing transition metal oxide, and the negative electrode is made of an electrode composed mainly of a carbon material capable of doping and undoping lithium ions, deterioration is remarkable when quick charge and discharge cycles are carried out as compared with a case where mild charge and discharge cycles are carried out. One of the main causes is deterioration by an oxidation-reduction reaction due to the charge and discharge of the lithium-containing transition metal oxide as the active material of the positive electrode.
Whereas, with the secondary power source of the present invention, when an adequate amount of the lithium-containing transition metal oxide is contained in the positive electrode, the activated carbon will be involved in the case of quick charge and discharge with a heavy current, and the lithium-containing transition metal oxide will be involved in the case of charge and discharge with a relatively small current. Accordingly, the lithium-containing transition metal oxide of the positive electrode is less burdened, whereby the deterioration due to charge and discharge cycles can be reduced to a low level, and it is possible to obtain a secondary power source having a high voltage, a high capacity and a long useful life for charge and discharge cycles.
Further, if the amount of the lithium-containing transition metal oxide contained in the positive electrode in the secondary power source of the present invention is reduced, only the activated carbon will substantially be involved in the charge and discharge irrespective of the current level. In such a case, the substantial role of the lithium-containing transition metal oxide will be to provide lithium ions to be doped in the carbon material of the negative electrode during the initial charge, and to supplement lithium ions when lithium ions in the electrolyte have decreased by the use of the secondary power source. Accordingly, the deterioration in,the capacity due to the charge and discharge cycles will be particularly small, although the capacity may be small as compared with a case where the content of the lithium-containing transition metal oxide is/large.
The amount of the lithium-containing transition metal oxide in the positive electrode is preferably from 0.1 to 80 wt %. If it is less than 0.1 wt %, the amount of lithium ions undoped in the initial charge tends to be inadequate relative to the amount of lithium ions which may be doped in the negative electrode, whereby the voltage of the secondary power source can not be made high. If it exceeds 80 wt %, the amount of activated carbon in the positive electrode will correspondingly be small, whereby the decrease in the capacity in the charge and discharge cycles will be large. Especially when a large capacity is of importance and a lithium-containing transition metal oxide is involved for the charge and discharge with a small current, the amount of the lithium-containing transition metal oxide is preferably from 20 to 70 wt %. Further, in order to increase the durability of the secondary power source by reducing the decrease in the capacity in the charge and discharge cycles, it is preferably from 0.1 to 15 wt % particularly preferably from 1 to 10 wt %.
The lithium-containing transition metal oxide to be contained in the positive electrode, is preferably a compound oxide of lithium and at least one transition metal selected from the group consisting of V, Mn, Fe, Co, Ni, Zn and W. Particularly preferred is a compound oxide of lithium and at least one member selected from the group consisting of Mn, Co and Ni. More preferred is LixCoyNi1xe2x88x92yO2 or LizMn2O4, wherein 0 less than x less than 2, 0xe2x89xa6yxe2x89xa61, and 0 less than z less than 2.
The activated carbon contained in the positive electrode preferably has a specific surface area of from 800 to 3,000 m2/g, more preferably from 900 to 2100 m2/g. The starting material and the activation conditions for the activated carbon are not particularly limited. For example, the starting material may be a phenol resin or a petroleum coke, and the activation method may, for example, be a steam-activation method, or a molten alkali activation method. Particularly preferred is activated carbon obtained by steam activation using, as the starting material, a coconut shell or a phenol resin. In order to reduce the resistance of the positive electrode, it is preferred to incorporate conductive carbon black or graphite as a conductive material to the positive electrode. In such a case, the conductive material is preferably from 0.1 to 20 wt % in the positive electrode.
As a method for preparing the positive electrode assembly, a method may, for example, be mentioned wherein polytetrafluoroethylene is mixed and kneaded, as a binder, to a mixture comprising an activated carbon powder and a lithium-containing transition metal oxide powder, followed by forming into a sheet shape to obtain a positive electrode, which is then bonded to a current collector by means of a conductive adhesive. Otherwise, an activated carbon powder and a lithium-containing transition metal oxide powder may be dispersed in a varnish having polyvinylidene fluoride, polyamideimide or polyimide dissolved as a binder, and the dispersion may be coated on a current collector by e.g. a doctor blade method, followed by drying. The amount of the binder contained in the positive electrode is preferably from 1 to 20 wt % from the viewpoint of the balance between the strength of the positive electrode assembly and the properties such as the capacity.
In the present invention, the carbon material capable of doping and undoping lithium ions preferably has a spacing of [002] of from 0.335 to 0.410 nm, particularly preferably from 0.335 to 0.338 nm, as measured by X-ray diffraction. The carbon material having a spacing exceeding 0.410 nm, tends to deteriorate in the charge and discharge cycles. Specifically, a material obtained by heat treating petroleum coke, a meso phase pitch-type carbon material or a gas-phase grown carbon fibers at a temperature of from 800 to 3,000xc2x0 C., natural graphite, artificial graphite or a hard (hardly graphitizable) carbon material, may, for example, be mentioned. In the present invention, any one of such materials may be preferably employed.
In the case where a carbon material obtained by low temperature treatment of e.g a hard carbon material or a petroleum coke, is to be used, it is preferred to use it in admixture with a graphite type carbon material such as a material obtained by graphitizing e.g. a gas phase grown carbon, whereby the resistance can be reduced. In such a case, the weight ratio of the hard carbon material or the petroleum coke to the graphite type carbon material is preferably from 95:5 to 70:30. If the graphite type carbon material is less than 5%, no adequate effect for reducing the resistance can be obtained, and if it exceeds 30%, the capacity of the negative electrode tends to be low.
The negative electrode assembly in the present invention can be obtained in such a manner that polytetrafluoroethylene is mixed and kneaded as a binder with a material capable of doping and undoping lithium ions, in the same manner as for the layer containing the activated carbon, followed by forming into a sheet shape to obtain a negative electrode, which is then bonded to a current collector by means of a conductive adhesive. Otherwise, a method may be employed wherein polyvinylidene fluoride, polyamideimide or polyimide is used as a binder, and the above-mentioned carbon material is dispersed in a solution having the resin for the binder or its precursor dissolved in an organic solvent, and the dispersion is coated on the current collector, followed by drying. These methods are both preferred.
In the method for coating the negative electrode layer on the current collector, the solvent for dissolving the resin for the binder or its precursor, is not particularly limited. However, N-methyl-2-pyrrolidone (hereinafter referred to as NMP) is preferred, since it is readily available and capable of readily dissolving the resin constituting the binder or its precursor. Here, the precursor for polyamideimide or the precursor for polyimide means one which will be converted to polyamideimide or polyimide, respectively, by polymerization under heating.
The binder obtained as described above will be cured by heating and is excellent in the chemical resistance, the mechanical properties and the dimensional stability. The temperature for the heat treatment is preferably at least 200xc2x0 C. At a temperature of at least 200xc2x0 C., even the precursor of polyamideimide or the precursor of polyimide will usually be polymerized to form polyamideimide or polyimide, respectively. The atmosphere for the heat treatment is preferably an inert atmosphere of e.g. nitrogen or argon or under a reduced pressure of at most 1 torr. The polyamideimide or the polyimide is resistant to the organic electrolyte to be used in the present invention and also has adequate resistance against heating at a high temperature at a level of 300xc2x0 C. to remove moisture from the negative electrode or against heating under reduced pressure.
In the present invention, an adhesive layer made of polyamideimide or polyimide may be interposed between the negative electrode and the current collector to further increase the bond strength between the negative electrode and the current collector. In such a case, a varnish having polyamideimide, polyimide or a precursor thereof dissolved in a solvent, may preliminarily be coated on the current collector by e.g. a doctor blade method and dried to form an adhesive layer, and a negative electrode may be formed on the adhesive layer. Further, it is preferred to have a conductive material such as copper or graphite dispersed in the varnish to form the adhesive layer, since it is thereby possible to reduce the contact resistance between the negative electrode and the current collector. The varnish containing such a conductive material may also be interposed as a conductive adhesive between the layer containing activated carbon and the current collector in a case where the layer containing activated carbon is formed into a sheet shape.
The lithium salt contained in the organic electrolyte in the present invention is preferably at least one member selected from the group consisting of LiPF6, LiBF4, LiClO4, LiN(SO2CF3)2, CF3SO3Li, LiC(SO2CF3)3, LiAsF6 and LiSbF6. The solvent preferably contains at least one member selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, sulfolane and dimethoxyethane. An electrolyte comprising such a lithium salt and a solvent has a high upper limit voltage and a high electrical conductivity. The concentration of the lithium salt is preferably from 0.1 to 2.5 mol/l, more preferably from 0.5 to 2 mol/l.